1. Field of the Invention
The present invention relates to a thin, monopolar-type, direct methanol fuel cell and a portable computer having the same.
2. Description of the Related Art
A direct methanol fuel cell (DMFC) has been developed as a replacement for a secondary battery used to power a laptop computer or other computerized and/or electrical/digital device. The DMFC is a clean energy source designed to replace fossil energy, and has a high energy density and a high energy conversion rate. Since the DMFC is operable at an ambient temperature, and can be miniaturized and sealed, it has been proposed for use in various fields, such as pollution free automobiles, home generators, a mobile communication systems, medical equipment, military equipment, and space equipment.
The DMFC produces electricity through an electrochemical reaction between methanol and oxygen. As depicted in FIG. 1 a unit cell of the DMFC, i.e., a fuel cell, comprises an anode 2 and a cathode 3, and a hydrogen ion exchange membrane arranged between them. The anode 2, the cathode 3, and the hydrogen ion exchange membrane 1 constitute a Membrane Electrode Assembly (MEA). The hydrogen ion exchange membrane 1 is formed of a polymer electrolyte having a thickness in a range of 50˜200 μm. The anode 2 and cathode 3 comprise supporting layers for supply and diffusion of fuel, and have catalyst layers for oxidation/reduction reaction of the fuel and oxygen.
The supporting layers of the anode 2 and the cathode 3 are formed of carbon paper or carbon fiber, and are wet-proofed for easy fuel supply and easy dissipation of reaction products.
At the anode 2, supplied fuel such as methanol, ethanol, or isopropyl alcohol reacts with water producing hydrogen ions, electrons, and carbon dioxide (oxidation reaction). The produced hydrogen ions flow to the cathode 3 through the hydrogen ion exchange membrane 1. At the cathode 3, hydrogen ions react with oxygen to produce water (reduction reaction).
Each reaction can be described as follows,CH3OH+H2O═CO2+6H++6e31 (Anode reaction)   [Reaction 1]3/2O2+6H++6e−=3H2O(Cathode reaction)   [Reaction 2]CH3OH+3/2O2=2H2O+CO2 (Overall reaction)   [Reaction 3]
A theoretical voltage of approximately 1.2 V can be generated by a unit cell of a DMFC. However, an open circuit voltage at an ambient temperature and at atmospheric pressure falls below 1 V due to a voltage drop caused by an active overpotential and a resistance overpotential. In reality, an actual operating voltage lies in the range of 0.4˜0.6 V. Therefore, to obtain higher voltages, a plurality of unit cells connected in series is required.
Typically, a bipolar stacking method is adapted for connecting the unit cells in series. However, this method increases the thickness of a portable computer due to the thick thickness of the fuel cell stack. Therefore, when a portable computer adapts a DMFC, the DMFC can be separated from the portable computer, and connected by a docking method as disclosed in Japanese Publication No. 2002-32154. The docking system requires pumps for supplying fuel from a fuel tank to the DMFC.
In order to be able to build in a thin portable computer, the DMFC should be thin. For this purpose, In one embodiment the DMFC of the present invention has a thin configuration which uses a monopolar plate.